1. Field of the Invention
The present invention relates to a semiconductor device and a data processing system including the same, and more particularly relates to a semiconductor device capable of adjusting a through rate of an output buffer, and a data processing system including the semiconductor device.
2. Description of Related Art
In semiconductor devices having a high data transfer rate such as a DRAM (Dynamic Random Access Memory), some devices are configured to be capable of changing a through rate of an output buffer at a manufacturing stage. Specifically, a selection circuit to select a through rate of the output buffer is provided in a chip beforehand, and data is written into the selection circuit to obtain an optimum through rate based on a result of an operation test performed in a wafer state. With this arrangement, the through rate of the output buffer is adjusted at the manufacturing stage. A general-purpose DRAM and the like are sometimes configured to select one through rate from among plural through rates prepared according to usage.
However, because the selection of through rate is irreversibly performed by using a fuse element or the like, once a through rate is selected, the through rate cannot be changed thereafter. The through rate obtained in practice dynamically changes depending on a power source voltage, environmental temperature, and secular changes and so on at an actual using time. Therefore, when the through rate is fixed at a manufacturing stage, there is a risk that the through rate is deviated from a design value.
As a method of solving this problem, Japanese Patent Application Laid-open No. H8-97693 discloses a method of measuring a through rate by using a ring oscillator, and setting variable a through rate of an output buffer at an actual using time based on a result of the measurement. Further, Japanese Patent Application Laid-open No. H7-86900 discloses a method of measuring a through rate by using two delay chains having different loads, and setting a through rate of an output buffer variable at an actual using time based on a result of the measurement.
However, the methods disclosed in Japanese Patent Application Laid-open Nos. H8-97693 and H7-86900 have a problem such that not only a chip area is substantially increased but also large power is consumed to measure a through rate, because both methods use exclusive circuits (a ring oscillator and delay chains) to measure a through rate. In addition, because the through rate is adjusted by a result of measurement made by exclusive circuits, changes of characteristics generated in the output buffer are not always reflected, and thus it is difficult to obtain a correct through rate.